There is conventionally known an image sensing apparatus which uses an image sensor capable of switching between interlaced read operation and frame read operation. By using this image sensor, interlaced read operation can attain image signals with high time resolution in moving image sensing, and frame read operation can attain still images without any time lag between fields in still image sensing.
The schematic arrangement of an image sensing apparatus which comprises a moving image recording function and still image recording function will be described below with reference to FIG. 3.
FIG. 3 is a block diagram showing the schematic arrangement of the image sensing apparatus which comprises a moving image recording function and still image recording function. In FIG. 3, reference numeral 100 denotes an image sensing apparatus which comprises a moving image recording function and still image recording function; 101, an imaging lens which forms an object image on a light-receiving surface of an image sensor 103 (to be described later); 102, a stop which controls the amount of incident light; and 103, the image sensor, which performs photoelectric conversion for the object image formed on the light-receiving surface in accordance with the light amount of the object image to output charge signals. Note that examples of the image sensor 103 include, inter alia, a CCD sensor, CMOS sensor.
Reference numeral 104 denotes an image sensor driving circuit which controls a charge read method, charge accumulation time, and the like of the image sensor 103; 105, a sample/hold circuit which samples and holds a charge signal output from the image sensor 103; 106, a variable gain circuit which changes the gain of a signal output from the sample/hold circuit 105; 107, a stop control circuit which controls the stop 102; and 108, an image signal processing circuit which generates a luminance signal and chrominance signals from an output signal from the variable gain circuit 106, and generates and outputs moving and still image signals on the basis of the generated luminance signal and chrominance signals.
Reference numeral 109 denotes a system control circuit which controls the image sensor driving circuit 104, variable gain circuit 106, stop control circuit 107, and image signal processing circuit 108 and controls the entire image sensing apparatus 100; 110, a still image recording circuit which stores a still image signal output from the image signal processing circuit 108; and 111, a moving image recording circuit which stores a moving image signal output from the image signal processing circuit 108.
A recording mode selection switch 112 is used to select either moving image recording or still image recording and outputs the selected information to the system control circuit 109. A moving image recording trigger switch 113 is used to give instructions to start/end moving image recording and outputs moving image start/end command information to the system control circuit 109. A still image recording trigger switch 114 is used to give instructions to execute still image recording and outputs still image recording execution command information to the system control circuit 109. An image sensing mode selection switch 115 is used to select one of a plurality of image sensing modes such as a full automatic mode, shutter speed priority mode, aperture priority mode, and film speed priority mode and outputs the selected image sensing mode information to the system control circuit 109.
Note that the system control circuit 109 performs the following control such that image sensing conditions are set in accordance with an image sensing mode selected by the image sensing mode selection switch 115. First, the system control circuit 109 outputs driving amount information to the stop control circuit 107 such that the image signal processing circuit 108 outputs a luminance signal of appropriate level. Upon reception of this information, the stop control circuit 107 performs open/close control for the stop 102 such that the exposure amount is appropriate. The system control circuit 109 outputs charge accumulation time information to the image sensor driving circuit 104. Upon reception of this information, the image sensor driving circuit 104 performs electronic shutter speed control to control the exposure by charge accumulation time control. The system control circuit 109 outputs gain information to the variable gain circuit 106. Upon reception of this information, the variable gain circuit 106 performs gain control which controls the gain of a signal output from the image sensor 103 to control the signal level.
If the system control circuit 109 determines on the basis of information input from the recording mode selection switch 112 that a moving image recording mode is selected, it controls the image sensor driving circuit 104 such that the image sensor 103 performs interlaced read operation and controls the image signal processing circuit 108 to perform a process of generating moving image signals. The system control circuit 109 instructs the moving image recording circuit 111 to perform moving image recording.
Likewise, if the system control circuit 109 determines on the basis of the information input from the recording mode selection switch 112 that a still image recording mode is selected, it controls the image sensor driving circuit 104 such that the image sensor 103 performs frame read operation and controls the image signal processing circuit 108 to perform a process of generating still image signals. The system control circuit 109 instructs the still image recording circuit 110 to perform still image recording.
In the interlaced read operation in moving image recording, signals of each pair of adjacent lines of the image sensor 103 are added to each other and read out, and pair formation methods differ between fields. For example, each pair of the first and second lines, the third and fourth lines, . . . are added to each other and read out in an odd-numbered field while each pair of the second and third lines, the fourth and fifth lines, . . . are added to each other and read out in an even-numbered field.
In the frame read operation, signals on the even-numbered lines in the array of pixels of the image sensor 103 are read out in the even-numbered field (first read field) while signals on the odd-numbered lines in the array of pixels of the image sensor 103 are read out in the odd-numbered field (second read field). At this time, a signal of each line is read out without being added to a signal of the immediately preceding line or immediately following line, unlike the interlaced read operation. For this reason, if the image sensor 103 is left exposed after reading signals in the first read field, signal charges are further generated by photoelectric conversion and are accumulated in pixels in the second read field to be read out. This makes a difference in signal level between the even-numbered field and the odd-numbered field. Consequently, a luminance flicker or hue shift occurs to greatly degrade the quality of the photographed image.
To solve the above-mentioned problem, in frame read operation, the image sensor 103 needs to be shielded from incident light at a timing when the image sensor 103 ends charge accumulation. This light-shielding state must be continued until reading of signals on the even-numbered lines in the first read field period and reading of signals on the odd-numbered lines in the second read field period are completed.
To shield the image sensor 103 from incident light, e.g., a mechanical shutter which uses a stop mechanism is employed. Note that a stop mechanism comprises the stop 102 and stop control circuit 107 of FIG. 3. More specifically, upon reception of a light-shielding command from the system control circuit 109, the stop control circuit 107 completely closes the stop 102 to shield light, thereby realizing a function of a mechanical shutter.
However, since the operation time of a mechanical shutter is sufficiently longer than the minimum charge accumulation time of the image sensor 103, light shielding cannot be completed in an instant. If the exposure amount is calculated using a time period until the stop 102 is completely closed as the exposure time, part of light which comes incident during the light shielding operation is lost, thus causing a loss in exposure amount. This loss reduces the signal level of a still image obtained by the frame read operation to a level lower than an optimum signal level which is set by calculation, and still images are recorded at incorrect exposure (underexposure). Under the circumstances, light amount losses caused by mechanical shutter operation need to be corrected.
A method of correcting light amount losses caused by mechanical shutter operation in the image sensing apparatus 100 shown in FIG. 3 will be described with reference to FIG. 4.
FIG. 4 is a flow chart showing an example of a conventional method of correcting light amount losses caused by mechanical shutter operation in the image sensing apparatus 100 shown in FIG. 3. In the example of a correction method shown in FIG. 4, the image sensing apparatus 100 corrects light amount losses only by electronic shutter control. First, in step S401, the system control circuit 109 determines whether it detects a still image recording request which is issued upon operation of the still image recording trigger switch 114. If a still image recording request is detected (YES in step S401), the flow advances to step S402. The system control circuit 109 determines a correction amount of an electronic shutter required for light amount correction from the closing time information of the stop 102 (mechanical shutter) and the current electronic shutter speed information. The system control circuit 109 outputs the corrected electronic shutter speed information to the image sensor driving circuit 104, and the flow advances to step S403. If no still image recording request is detected (NO in step S401), the system control circuit 109 continues the detection operation in step S401 until it detects the request.
In step S403, the system control circuit 109 controls a closing start timing such that the stop 102 (mechanical shutter) ends closing operation at a predetermined timing. The system control circuit 109 issues a closing command to the stop control circuit 107 at a predetermined timing. Upon reception of this command, the stop control circuit 107 controls the stop 102 (mechanical shutter) so as to end the closing operation at the predetermined timing. After the stop 102 (mechanical shutter) is completely closed to shield light in step S403, the flow advances to step S404. The system control circuit 109 controls the image sensor driving circuit 104, thereby reading out charge signals from the image sensor 103 in an even-numbered field and odd-numbered field. The system control circuit 109 causes the image signal processing circuit 108 to generate a still image on the basis of the charge signals. Then, the still image recording circuit 110 stores the still image generated by the image signal processing circuit 108.
By the above-mentioned operation, the image sensing apparatus 100 can generate and record still images whose light amount losses caused by mechanical shutter operation are corrected.
Various image sensing modes are available in the image sensing apparatus 100, and any of them can be selected in accordance with a scene to be sensed. For example, image sensing modes such as a shutter speed priority mode in which the user selects or sets the electronic shutter speed and a film speed priority mode in which the user selects or sets the gain control information are known. The image sensing apparatus 100 further has a function of setting individual image sensing conditions such as the shutter speed and exposure amount to suit the preferences of the user.
However, since light amount losses caused by mechanical shutter operation are conventionally corrected without regard to the above-mentioned image sensing modes, image degradation may occur. For example, assume that the user selects a high shutter speed in the shutter priority mode. In this case, if the system control circuit 109 performs exposure correction by controlling the electronic shutter to operate at a low speed to correct light amount losses caused by mechanical shutter operation, a greatly blurred object image is obtained. Likewise, assume that the user selects a low film speed in the film speed priority mode. In this case, if the system control circuit 109 performs exposure correction by gain control to correct light amount losses caused by mechanical shutter operation, an image with a low S/N ratio is obtained.